1. Field of the Invention
The present invention relates to a gaseous-impurities capturing method which is used to capture gaseous impurities in a semiconductor device manufacturing process, and an apparatus for manufacturing a semiconductor device by using the capturing method.
2. Description of the Related Art
Recently, higher integration design has been required for semiconductor devices such as semiconductor memories (for example, dynamic RAM (hereinafter referred to as DRAM), etc., and in order to meet this requirement, the area which is indispensable to each memory cell has been extremely reduced. For example, in the case of 1 MDRAM or 4 MDRAM, a design rule for providing the minimum design width of 0.8 micrometer is adopted. On the other hand, in the case of 16 MDRAM, a design rule for providing the minimum design width of 0.6 micrometer is adopted. As the area of the memory cell is reduced as described above, the amount of charges accumulated in the memory cell is also reduced, and thus it is difficult to ensure the charge amount which is required for the memory cell in accordance with the high integration design.
On the other hand, in order to ensure the charge amount required for the memory cell, a memory cell having a trench type or laminate type capacitor has been proposed and used in practice.
A memory cell structure having a laminate type capacitor has an advantage that it has higher resistance to a soft error than a memory call structure having a trench type capacitor, and also an advantage that it causes no damage to a silicon substrate. Therefore, the memory cell structure having the laminate type capacitor has been expected as a memory cell structure of the next generation. Further, it has been considered that the resistance to alpha ray is enhanced by designing a trench capacitor in the laminate type trench structure. Accordingly, the laminate type memory call is promising as a next generation technique.
In such a situation, there has been proposed an application of an HSG (hemi-spherical-grain) technique to a laminate type capacitor which is applicable to DRAMs of 64 Mbytes or more (as disclosed in Japanese laid-open Patent Application No. Hei-8-306646). According to the HSG technique, a large number of hemi-spherical grains or mushroom-shaped grains are formed on the surface of a storage electrode of a capacitor to substantially increase the surface area of the storage electrode and thus achieve a large capacity.
When the storage electrode as described above is formed, silicon-contained gas such as SiH.sub.4 or the like is irradiated onto a deposited amorphous silicon layer to arrange silicon atoms serving as nucleuses on the layer, and after the irradiation, the silicon atoms located around the nucleuses are collected with the thus-formed nucleuses at the center, whereby uneven particles, that is, hemi-spherical or mushroom-shaped grains are formed on the surface. The film having the hemi-spherical or mushroom-shaped grains becomes a polysilicon film as a result, and in the following description, the layer having the hemi-spherical or mushroom-shaped grains is hereinafter referred to as "HSG-Si film".
In the case where the hemi-spherical grains are formed on amorphous silicon as described above, if a natural oxide film of several atom layers or the like is formed on the surface of amorphous silicon, the oxygen atoms of the natural oxide film disturb migration of Si atoms, and thus disable the formation of hemi-spherical grains. Actually, a silicon oxide film as a natural oxide film is formed on amorphous silicon even under such a condition that the partial pressure of oxygen is equal to about 1.times.10.sup.-6 Torr. Therefore, impurities such as oxygen, water, etc. must be prevented from remaining in the atmosphere during the process of forming the hemi-spherical grains.
In order to solve the above problem, the Japanese patent application described above discloses a method of treating a natural oxide film formed on amorphous silicon with hydrogen fluoride solution to remove the natural oxide film, and then putting a wafer in an apparatus having a load lock chamber to form an HSG-Si film.
However, in the case where many wafers are put into a reaction chamber (formed of quartz, SiC or the like), which is evacuated by an vacuum pump and heated, through the load lock chamber, even when the natural oxide films on the wafers are temporarily removed by the hydrofluorination, it is confirmed that natural oxide films are formed on amorphous silicon again and impurities such as water, etc. remain in the heated reaction chamber. This is because when the wafers are put into the reaction chamber, water which is adsorbed at the inside of and on the surface of the silicon oxide film formed on the wafer is heated and separated from the silicon oxide film. If the number of wafers is small, the effect of water, oxygen and organic materials which are separated from an interlayer film, etc. is small. However, if many wafers are put into the reaction chamber, the amount of the impurities is increased, and thus it is difficult to keep the Si film surface clean. Particularly, the increase of the partial pressure of water has a strong effect on the formation of HSG. This is because the water separated from the silicon oxide film promotes oxidation of the electrode surface on the wafer.
As described above, it has been found that when the natural oxide film remains on amorphous silicon, silicon atoms do not sufficiently gather around the atoms serving as nucleuses even when silicon-contained gas is irradiated, and thus the surface area increase rate of the HSG-Si film has a limitation. This is because the natural oxide film disturbs the migration of silicon atoms on the surface of amorphous silicon. Further, once the electrode surface is oxidized, the surface area increase rate of the HSG-Si film is not enhanced even when the vacuum degree of the load lock chamber and an anneal chamber is set to about 1.times.10.sup.-8 Torr by a turbo pump or the like.
Japanese Laid-open Patent Application No. Hei-5-206046 proposes that an impurities-capturing function is provided by a reactor core pipe itself which is used in a heat treatment step of a semiconductor manufacturing process. According to this technique, the reactor core pipe is formed of polysilicon, and a heat treatment which causes the impurities of polysilicon itself to gather at the upper end portion of the reactor core pipe is performed when the reactor core pipe is formed.
This publication does not indicate the problem that the surface area increase rate for the formation of the HSG-Si film has a limitation. Further, this publication does not suggest that the reactor core pipe is used to form the HSG-Si film.
Further, since the impurities capturing capability is provided to the reactor core pipe itself, even if the reactor core pipe of this publication is applied to form hemi-spherical grains, the capturing capability of the reactor core pipe is reduced due to adhesion of impurities every time the reactor core pipe is used. Therefore, it is required to exchange the reactor core pipe by a new one or treat the reactor core pipe at high temperature again. Accordingly, this reactor core pipe is unsuitable for repetitive use.
Next, referring to Japanese Laid-open Patent Application No. Hei-1-197388, this publication proposes that high-purity getter material which can adsorb and absorb residual impurities under vacuum is attached in a growth chamber of a molecular-beam crystal apparatus. In this publication, high-purity Al, Mn, Nd, Sc, Sm and Yb are suggested as the capture material. As described above, this publication indicates that metal capture material which is formed of Al or the like is suitable to capture residual impurities such as H.sub.2 O, CO, CO.sub.2, etc.
However, such metal capture material is gasified and vaporized by a high-temperature heat treatment at 500.degree. C. or more, and thus it is not applicable to a semiconductor device manufacturing process in which the overall reaction chamber is subjected to the high-temperature heat treatment. Further, the device characteristic is generally deteriorated if metal is attached to Si, and thus the metal capture material is unsuitable for the device manufacturing.
Actually, when the HSG-Si film is formed by the HSG technique, amorphous silicon is usually subjected to the heat treatment at, a high temperature of about 550.degree. C. in an anneal chamber of an LPCVD apparatus (that is, a reaction chamber which is reduced in pressure and kept at high temperature) to be annealed, thereby forming an HSG-Si film. As described above, when the HSG-Si film is formed, the high-temperature heat treatment of about 550.degree. C. is required, and thus the metal capture material as disclosed in the above publication is not usable as getter material in the HSG-Si film forming process.